Water mains, oil pipelines and the like are usually buried after being completed. After being completed, they are opened and materials such as water or oil are transported in the pipeline. After a period of time, it sometimes becomes necessary to extend the pipeline or to add fittings at various points along the pipeline. When this becomes necessary, it is desirable that the fittings can be added without the necessity of shutting down the entire pipeline. Thus, if a subdivision requires a branch connection from a water main, it is extremely inconvenient for other users of the water main to go without water while a branch is being put in.
In order to solve this problem contractors commonly employ special types of tapping machines. Initially, the pipeline or water main is exposed by excavation and a branch connection is welded or otherwise connected thereto. The branch is positioned directly over a sealed portion of the pipeline. After the branch is connected, a gate valve is sealingly attached to the branch. Once the gate valve is attached, a tapping machine is attached to the gate valve opposite the branch. The tapping machine has a portion which is liquid tight which is attached to the gate valve. The gate valve is then left open and the tapping machine cuts through the pipe and draws a coupon of metal through the gate valve, whereupon the gate valve is then closed, the tapping machine and the coupon of metal are removed and the tap into the pipe has been completed without the necessity of shutting off fluid flow through the pipe.
While the above mentioned method is used for tapping into large mains, pipelines, tanks or flat wall surfaces, there are still problems that are encountered with the tapping machines which are used in the process. One of the main problems encountered with the older tapping machines is that the cutting portion of the tapping machine is commonly driven from a worm gear drive. A worm gear connection is among the most inefficient mechanical connections which can be used. The amount of power wasted through a worm gear is approximately 50% of the input power. For small cuts made in small pieces of pipe, input power wasted through such a worm gear drive is not significant; however, when tapping into a large pipeline or large water main, from which it is not uncommon to remove a 36 inch diameter or larger coupon, 50% power losses through the worm gear drive train can be quite significant and can both serve to increase the amount of cutting time needed and require larger capacity power sources to drive the cutter.
Another problem which is often encountered with present day tapping machines, is that the modern tapping machines use an automatic feed arrangement which is completely enclosed and is not adjustable. The automatic feed arrangement advances the cutter into engagement with the pipe which is being cut. Many automatic feed arrangements employ a stationary drive assembly having a feed screw which rotates with a boring bar; and a feed nut which rotates at a slight differential rotational velocity with respect to the feed screw. During conditions of heavy loading, the relative velocity of the feed screw and the feed nut cannot change, thereby causing jamming of the cutter. In addition, if the feed screw is being heavily loaded, this can cause the feed screw to break while the operator is unaware of the fact and lead to great expense on the job, because of the need to shut down the job and rebuild the tapping machine.
What is needed then is a tapping machine having an efficient drive train with a minimum of power loss through the drive train. The tapping machine should also have an open construction and an easily controllable feed mechanism to prevent jamming of the cutter and possible breakage of the feed mechanism.